Perhaps it wasn’t quite the right formulation when I said “the story the author originally set out to tell us”, maybe it would better to talk about the “core story”. But I think you are confounding story with reception somewhat. There’s no doubt that books 5 to 12 (Odysseus’ fantastic adventures) have been immensely popular for over two millennia, probably more so than the rest of epic, but the fact remains that they are only a small part of the Odyssey, which I think justifies my calling the digressions. By far the biggest part of the epic is set in Ithaca; it’s much too long to be called just a frame story, so I call it the main story, and judging just by length I suppose it’s legitimate to at least suspect that it was the part that interested the poet most. If you don’t want to call the Cyclops and Circe etc. digressions I’m fine, but I don’t think it’s too bad in lack of a better word.
Homeric epics aren’t “novels”, but they certainly have a plot, which is a very important and integral part of them. Both the Iliad and the Odyssey are very carefully constructed coherent wholes; I would say both (but especially the Iliad) have an overall structure that is simply brilliant. Just read them starting from line 1, and it should be evident. If you read epics simply as disconnected episodes, or for “poetry and wordplay”, you are missing a large part. There’s a paradox here, because people didn’t really read alone like they do now, and the works are too long for a single day’s oral performance, so the question remains why they bothered to make such long, carefully built stories, when the possibilities to perform them would have been few and far between.
As for whether the Iliad or the Odyssey have an “original author”, that’s a thorny question, but I think the answer is firmly YES. It is complicated, and very different opinions and theories have been formulated over centuries. You are probably going to retort that the Homeric epics are oral poetry, and oral poetry is by nature fluid and doesn’t have an original author – I think yes and no. I believe the epics were both composed in writing (they are long TEXTs, remember), but by someone who was very well versed with the oral tradition. The subject matter is traditional, but what we have is clearly someone’s version of a traditional story - something original, an individual of art. A book called The Making of the Iliad by Martin L. West (probably one the greatest Greek scholars who ever lived) makes this point, and I’ve decided to believe him at least in the main part.
You might want to have a look at this old thread about the subject: https://www.textkit.com/t/silent-expurgation/
As for the moral message of the Odyssey (let’s leave the Iliad out for now, which is a slightly different beast), I think it’s pretty important but of course not the kind of moral that we’d expect. The themes of regret and self-doubt are in the film The Return, but they are part of what I called “modern sensibilities” in my first post, and are not in the original epic. I don’t disagree with ClassyCuss about the epic’s moral message here, but I think there are other moralistic themes that also important. One is the theme of hospitality – throughout the Odyssey, we have examples of what it is to be a good host (Telemachus, Nestor, Menelaus, Eumaios) or a bad host (the prominent example is the Cyclops, but others as well). Perhaps the case in point are the suitors – they crime they really get their just deserts for is abusing hospitality. Another moralistic aspect is that the Odyssey is strongly supportive of existing social order: everyone should know their place, slaves should be loyal to their master etc. (just look at the loyal Eumaios, and compare to what happened to the unloyal handmaidens) – no wonder, as the poet was probably sponsored by some wealthy patron.